The Greenlee tester has some value, and is somewhat helpful to me. This is especially the case when the bulb / ballast in question is unusual or exensive enough that it's not worth it to stock spare ones.
The tester, however, is somewhat subjective in use, and you need a dark place for testing. For your usual fluorescent fixtures, simply putting in known "good" bulbs -usually from a working fixture nearby- is the simplest, most reliable solution. Good bulbs, good power to the fixture....not much left but the ballast!
Now- to belabor the obvious- many places start losing ballasts in rapid succession as the ballasts reach their "design age." Be alert for this pattern; if the old ballasts are about 10 years old, perhaps it's time to talk to the customer about replacing with T-8's as they wear out.
